The invention relates to a sequential method and system to compound heat-vulcanizable silicone compositions.
A heat-vulcanizable silicone composition comprises a high viscosity silicone polymer, an inorganic reinforcing filler and various additives that aid processing or impart desired final properties to the composition. A vulcanizing agent can be added and the composition heat-cured to fabricate silicone rubber moldings such as gaskets, medical tubing and computer keypads.
Typically, the heat-vulcanizable silicone composition is produced by kneading a high-viscosity polydiorganosiloxane, the inorganic filler and additives by means of a batch kneading machine such as a high intensity Banbury mixer or a low intensity double arm dough mixer. In this process, polydiorganosiloxane, inorganic filler and treating agents are batch mixed until desired properties are obtained. This process requires long residence times and large amounts of energy. Non-homogeneous shear and extensional stresses across a commercial sized batch can result in non-uniform size distribution of filler that results in variations in properties. Batches processed at different times may be characterized by different physical properties. The batch process is labor, energy and capital intensive and produces materials of only marginal consistency.
In Kasahara et al., U.S. Pat. No. 5,198,171, a premix is formed in a first step wherein a polydiorganosiloxane having a viscosity at 25xc2x0 C. of 1xc3x97105 cP or more, an inorganic filler and a treating agent are mixed in a high-speed mechanical shearing machine. The step produces a flowable particulate mixture in which each ingredient is present in a substantially uniform, finely dispersed state. The premix is then fed at a constant feed rate into an extruder that has two screws rotating in the same direction and has a length to diameter ratio (L/D) of 25 to 50.
In Hamada et al., U.S. Pat. No. 5,409,978, a premix of polydiorganosiloxane, inorganic filler and treating agents is formed at a temperature in the range of about 200xc2x0 C. to 300xc2x0 C. in a co-rotating continuous double screw extruder with L/D of about 25 to 50. The premix is then compounded and heat treated at 150xc2x0 C. to 300xc2x0 C. in a counter-rotating, double screw extruder. A useful L/D ratio for the second extruder is in the range of about 5 to 15.
Highly vigorous first step processes for forming a premix of silicone polymer, filler and treating agent generate a product having a high volatiles content. The premix must be mixed with additional polymer and devolatilized in a second step to produce a useful product. Other newer processes that avoid a premix and/or that use a raw untreated filler can be limited in throughput. Larger diameter extruders will increase throughput. However, larger diameter extruders must be custom built. Hence, larger extruders are expensive and cost more per unit capacity. There is a need for a process that continuously and consistently produces a devolatilized high viscosity filled silicone polymer composition at high throughput. Further, there is a need for an improved continuous compounding method that efficiently utilizes compounding equipment while providing commercial scale production.
The invention provides a method and system that efficiently compounds high levels of inorganic filler, processing fluid and silicone polymer at a commercial rate into homogeneous filled and devolatilized silicone compositions. In the method, filled silicone compositions are compounded by compounding a filler, processing fluid and silicone polymer in a first compounding apparatus to produce a first dispersed composition. Simultaneously, filler, processing fluid and silicone polymer are compounded in a second compounding apparatus that shares a common extruder shaft with the first compounding apparatus to produce a second dispersed composition.
The system comprises a first compounding apparatus and a second compounding apparatus that shares a common shaft with the first compounding apparatus.
In another embodiment, the invention relates to an extruder transition section. The section comprises an enclosed discharge chamber defined by a first sectioning wall, a second sectioning wall and a contoured lower wall that transitions toward a discharge port. A shaft extends through the first sectioning wall, traverses the chamber and extends through the second sectioning wall.